Detent devices are used to provide tactile feedback to users of rotary switches. These enable a user to determine whether and how far a switch has been turned. In addition, such devices prevent inadvertent rotation of a switch by requiring a threshold amount of torque to overcome the resistance of the device. This torque requirement also reduces the risk that a switch will change positions due to shocks or vibrations in a hostile environment. Known devices typically use a stylus that is perpendicularly biased against a detented surface.
In addition, detent devices are used to select discrete switch positions and to prevent continuously variable intermediate positioning. Detent positions are typically consistent and repeatable.
A major problem in prior art devices is that wear of the detented surface reduces the useful life, and degrades the tactile feel performance throughout the product life. Reduced internal spring forces will permit marginally increased lifespan, but at the cost of degrading initial tactile feel. There is generally a desire in the market for improved tactile feel.
Known devices typically use a spring biased stylus, such as a pair of balls, that is pressed against a moving surface formed with alternating hills and valleys. The stylus sweeps out a constant circular path, wearing down the hills at a much more rapid rate than that of the slow-wearing valleys. Consequently, the torque required to rotate the device is progressively reduced, and tactile feel is degraded.
One such device is a SDB161 "Slimline" potentiometer manufactured by Noble, Inc. It employs a metal leaf spring having a protruding dimple that follows hills and valleys molded on a plastic rotary hub. The hills wear down to the level of valleys as the dimple abrades the hub. Wearout typically occurs at about 300,000 revolutions.
A second type of device is a "Series 61" device manufactured by Grayhill, Inc., and is illustrated schematically in FIG. 1. The SG-OE1-16 device manufactured by Standard Grigsby, Inc. is comparable. These devices each comprise a pair of oppositely biased balls that follow an interior circumferential surface formed with inwardly convex circular bumps that function as hills.
Twin ball devices such as the FIG. 1 device have the disadvantage that any misalignment that leaves one ball fully detented in a valley while the other is not causes the balls to fight each other, reducing the stability of the detented position and consequently preventing a crisp feel. In addition, the bumps of the FIG. 1 device wear down continually throughout the life of the device, progressively degrading tactile performance. Such a device typically lasts about 1,000,000 revolutions.
FIG. 2 shows a third type of detent device. The Hewlett-Packard Company HP 17501A channel indicator includes a switch using a twin ball design in an application where an extended life span is not required. The FIG. 2 device presses the balls perpendicularly against a perforated plate.
A central feature of the preferred form of the present invention is that a detent ball is biased by a spring at a non-perpendicular angle to a detented hub surface. Therefore, as the hub wears, the ball gradually progresses radially inward to encounter new surface material.
A second feature provides that the indentations are sufficiently deep so that the ball does not touch the bottom of the indentations. This permits the device to retain a crisp feel throughout a long product life.
A third feature of the preferred form of the invention is that it uses only a single ball as a stylus, thereby avoiding the disadvantages of twin ball devices.
A fourth feature of the preferred form of the invention is that the ball freely rolls over the detent hub, and does not slide over the surface. This eliminates wear caused by abrasion.
Increased use of the preferred embodiment does not degrade device torque or tactile feel. In fact, these properties have actually been found to improve slightly over time in the preferred form of the invention because the rate of wear of the hub surface (functioning as hills between the indentations) is equalled or exceeded by the wear rate of the indentations. The indentations wear faster because the ball is wedged progressively deeper into each indentation over time, but does not hit bottom. This wedging process microscopically deforms the rim of an indentation, forcing the rim outward and upward to buttress the adjacent hub surface, thereby counteracting the effect of wear.
The amount of work the ball must perform on the spring as the device is switched to the adjacent detent is determined by the amount of vertical travel by the ball between the detented position (ball in an indentation) and the non-detented position (ball on the hub surface between indentations). Therefore, the torque and tactile feel are substantially undegraded because the vertical travel distance is maintained or increased as the device wears. The device wears out when ball path is sufficiently worn so that the spring crashes into the hub surface.
As a result of these and other features, the present invention provides a rotary tactile feedback device which does not degrade over its life, and which has a typical lifespan of 3,000,000 revolutions, a factor of ten improvement over comparable existing devices.